[0001] The present invention relates to a multi-channel inverter for a photovoltaic apparatus.
Multi-channel inverters are widely used in photovoltaic apparatuses including multiple
photovoltaic strings.
[0002] A multi-channel inverter generally comprises a DC section including a plurality of
DC input channels receiving DC electric power from corresponding photovoltaic strings,
an DC/AC conversion section providing a DC/AC conversion of DC electric power provided
by the photovoltaic strings and an AC section providing AC electric power to an electric
power distribution grid, for example the mains.
[0003] When the inverter is capable of carrying out MPPT (Maximum Power Point Tracking)
functionalities, each DC input channel typically includes a DC/DC converter adapted
to receive an input DC voltage from a corresponding photovoltaic string and provide
an output DC voltage having a controllable voltage value to the DC/AC conversion section.
[0004] As is known, many types of multi-channel inverters are not provided with electric
isolation arrangements (e.g. an electric transformer) adapted to galvanically isolate
the DC section and the AC section.
[0005] In these cases, current international regulations lay down that the resistive impedance
towards ground (normally referred to as "isolation resistance") of the photovoltaic
strings has to be cyclically measured under suitable test conditions of the inverter.
[0006] In most of the cases, the isolation resistance is measured by providing a suitable
detection circuit between the input terminals of the DC/AC conversion section and
the ground.
[0007] In inverters providing MPPT functionalities, the above-mentioned technical solution
is applicable only when the DC input channels have their positive or negative electric
lines substantially equipotential.
[0008] However, such a technical solution cannot be adopted when DC/DC converters having
a double-level configuration are arranged at the DC input channels to provide relatively
high output voltages (i.e. higher than 1 kV) to the DC/AC conversion section. In this
case, in fact, diodes are present on both the electric lines of each DC input channel
and, consequently, the DC input channels cannot have their positive or second electric
lines equipotential.
[0009] In order to solve this problem, various measuring circuit arrangements have been
proposed in the state of the art, some examples of which are described in
CN106603007B.
[0010] Unfortunately, the experience has shown that currently available solutions do not
yet ensure accurate and reliable measurements of the isolation resistance of the photovoltaic
strings. Additionally, it is not possible to preventively check whether active components
(switches) in the designed measuring arrangements are correctly operating when the
isolation resistance measurements are required.
[0011] Of course, the above-mentioned drawbacks may lead to unacceptable and potentially
dangerous errors in measuring the isolation resistance of the photovoltaic strings.
[0012] The main aim of the present invention is to provide a multi-channel inverter for
a photovoltaic apparatus, which enables the disadvantages described above to be overcome.
Within this aim, another object of the present invention is to provide a multi-channel
inverter, in which the isolation resistance can be measured in an accurate and reliable
manner even when the inverter is designed to provide MPPT functionalities and the
DC input channels of the inverter do not have any equipotential electric lines.
[0013] Another object of the present invention is to provide a multi-channel inverter, which
has a compact structure with a relatively small size.
[0014] Yet another object of the present invention is to provide a multi-channel inverter,
which is easy and inexpensive to produce at industrial level.
[0015] This aim and these objects, together with other objects that will be more apparent
from the subsequent description and from the accompanying drawings, are achieved,
according to the invention, by an inverter, according to claim 1 and the related dependent
claims.
[0016] Characteristics and advantages of the present invention will be more apparent with
reference to the description given below and to the accompanying figures, provided
purely for explanatory and non-limiting purposes, wherein:
- Fig. 1 schematically illustrates a photovoltaic apparatus including an inverter, according
to the present invention;
- Fig. 2 schematically illustrates a partial view of a two-channel inverter, according
to the present invention;
- Fig. 2A schematically illustrates a partial view of a multi-channel inverter, according
to the present invention;
- Figs. 3-6 schematically illustrate some aspects of the inverter, according to the
present invention.
[0017] With reference to the aforesaid figures, the present invention relates to an inverter
1 for a low voltage photovoltaic apparatus 100.
[0018] For the sake of clarity, it is specified that the term "low voltage" refers to operating
voltages lower than 1 kV AC and 2 kV DC.
[0019] Referring to figure 1, a photovoltaic apparatus 100 including the inverter 1 is shown.
The photovoltaic apparatus 100 comprises one or more photovoltaic strings 200, each
of which may comprise one or more photovoltaic panels.
[0020] The photovoltaic strings 200 are electrically connected with the inverter 1 that
in turns is electrically connected with an electric power distribution grid 300 (e.g.
the mains or a load circuit).
[0021] In general, the inverter 1 is adapted to receive DC electric power from the photovoltaic
strings, to convert said DC electric power into AC electric power and to provide said
Ac electric power to the electric power distribution grid.
[0022] The inverter 1 comprises a DC section 1A, an DC/AC conversion section 1B and an AC
section 1C.
[0023] The DC section 1A includes a DC bus electrically connected with the photovoltaic
strings 200 to receive DC electric power from these latter.
[0024] According to the invention, the inverter 1 is of the multi-channel type.
[0025] The DC section 1A thus comprises a DC bus formed by a plurality of DC input channels
CH
1, CH
2, CH
N, each of which has an input port adapted to be electrically connected with a corresponding
photovoltaic string 200 and an output port electrically connected with the DC/AC conversion
section 1B.
[0026] The DC/AC conversion section 1B is electrically connected between the DC section
1A and the AC section 1C.
[0027] Conveniently, the DC/AC conversion section 1B comprises an input port (also referred
to as "DC-link") electrically connected in parallel with all the output ports of the
DC input channels CH
1, CH
2, CH
N of the DC section 1A and an output port electrically connected with the AC section
1C
[0028] Preferably, the DC/AC conversion section 1B comprises one or more DC/AC converters
adapted to provide a DC/AC conversion of DC electric power provided by the photovoltaic
strings 200 into AC electric power.
[0029] The AC section 1C of the inverter 1 is electrically connected with the DC/AC conversion
section 1B and the electric power distribution grid 300.
[0030] Preferably, the AC section 1C comprises an input port electrically connected with
the output port of the DC/AC conversion section 1B and a suitable AC bus adapted to
deliver the AC electric power provided by the DC/AC conversion section 1B to the electric
power distribution grid 300.
[0031] In general, the DC/AC conversion section 1B and the AC section 1C of the inverter
1 may be of known type and will not be further described in further details for the
sake of brevity.
[0032] In figure 2, an embodiment of the inverter 1 having two DC input channels CH
1, CH
2 is shown while, in figure 2A, an embodiment of the inverter 1 having a generic number
of N (N > 2) DC input channels CH
1, CH
2, CH
N is shown.
[0033] Each DC input channel CH
1, CH
2, CH
N comprises an input port formed by a first input terminal IT+ (e.g. having a positive
voltage polarity) and a second input terminal IT- (e.g. having a negative voltage
polarity) electrically coupleable with output terminals of a corresponding photovoltaic
string 200.
[0034] Each DC input channel CH
1, CH
2, CH
N comprises an output port formed by a first output terminal OT+ (e.g. having a positive
voltage polarity) and a second output terminal OT- (e.g. having a negative voltage
polarity).
[0035] The output terminals OT+ and OT- of each DC input channel CH
1, CH
2, CH
N are electrically coupled respectively with third and fourth input terminals T+ and
T- (e.g. having a positive and negative voltage polarity, respectively) of the DC/AC
conversion section 1B, which form the input port of this latter section of the inverter
1. In this way, the output ports of the DC input channels CH
1, CH
2, CH
N are electrically connected in parallel with the input port of the DC/AC conversion
section 1B.
[0036] Each DC input channel CH
1, CH
2, CH
N comprises a first electric line L+ (e.g. having a positive voltage polarity) between
and including the first input and output terminals IT+ and OT+ and a second electric
line L- (e.g. having a negative voltage polarity) between and including the second
input and output terminals IT- and OT-.
[0037] According to the invention, the DC section 1A comprises a DC/DC converter CV
1, CV
2, CV
N for each DC input channel CH
1, CH
2, CH
N.
[0038] Each DC/DC converter CV
1, CV
2, CV
N is electrically connected between the input terminals IT+, IT- and the output terminals
OT+, OT- of the corresponding DC input channel CH
1, CH
2, CH
N.
[0039] Each DC/DC converter CV
1, CV
2, CV
N comprises one or more first components electrically connected in series with the
first input and output terminals IT+, OT+ of the corresponding DC input channel CH
1, CH
2, CH
N and are part of the first electric line L+ of said DC input channel.
[0040] Each DC/DC converter CV
1, CV
2, CV
N comprises one or more second components electrically connected in series with the
second input and output terminals IT-, OT- of the corresponding DC input channel CH
1, CH
2, CH
N and are part of the second electric line L- of said DC input channel.
[0041] According to some embodiments of the invention, said first components may include
at least a diode arranged in such a way to allow the circulation of a current along
the first electric line L+ with a flow direction going from the first input terminal
IT+ to the first output terminal OT+ and/or said second components may include at
least a diode arranged in such a way to allow the circulation of a current along the
second electric line L- with a flow direction going from the second output terminal
OT- to the second input terminal IT-.
[0042] Preferably, as shown in figures 2 and 2A, each DC/DC converter CV
1, CV
2, CV
N has a so-called "two-level boost configuration".
[0043] In the embodiments of figures 2 and 2A, each DC/DC converter CV
1, CV
2, CV
N comprises a first inductance L1 and a first diode D1 arranged along the first electric
line L+ of the corresponding DC input channel CH
1, CH
2, CH
N.
[0044] The first inductance L1 is electrically connected between the first input terminal
IT+ and a first intermediate node N1 of the first electric line L+ whereas the first
diode D1 has its anode electrically connected with the first intermediate node N1
and its cathode electrically connected with the first output terminal OT+.
[0045] In the embodiments of figures 2 and 2A, each DC/DC converter CV
1, CV
2, CV
N comprises a second inductance L2 and a second diode D2 arranged along the second
electric line L- of the corresponding DC input channel CH
1, CH
2, CH
N.
[0046] The second inductance L2 is electrically connected between the second input terminal
IT- and a second intermediate node N2 of the second electric line L- whereas the second
diode D2 has its cathode electrically connected between the second intermediate node
N2 and its anode electrically connected with the second output terminal OT-.
[0047] In the embodiments of figures 2 and 2A, each DC/DC converter CV
1, CV
2, CV
N further comprises an input capacitance C0 electrically connected in parallel with
the input terminals IT+, IT- of the corresponding DC input channel CH
1, CH
2, CH
N, a first switch S1 electrically connected between the first intermediate node N1
and a third intermediate node N3, a first output capacitance C1 electrically connected
between the first output terminal OT+ and the third intermediate node N3, a second
switch S2 electrically connected between the second intermediate node N2 and the third
intermediate node N3 and a second output capacitance C2 electrically connected between
the second output terminal OT- and the third intermediate node N3.
[0048] The first and second switches S1 and S2 (e.g. MOSFETs) of each DC/DC converter CV
1, CV
2, CV
N generally operate in response to first control signals (not shown) received in input.
In the embodiments of figures 2 and 2A, the third intermediate nodes N3 of the DC/DC
converters CV
1, CV
2, CV
N of all the DC input channels CH
1, CH
2, CH
N are electrically connected one with another in such a way to be equipotential.
[0049] As it may be noticed, in the embodiments of figures 2 and 2A, the first diode D1
and the first inductance L1 therefore constitute the above-mentioned first components
of the DC/DC converter CV
1, CV
2, CV
N, which are electrically connected in series with the first input and output terminals
IT+, OT+ of the corresponding DC input channel CH
1, CH
2, CH
N along the first electric line L+ of said DC input channel.
[0050] On the other hand, the second diode D2 and the second inductance L2 constitute the
above-mentioned second components of the DC/DC converter CV
1, CV
2, CV
N, which are electrically connected in series with the second input and output terminals
IT-, OT- of the corresponding DC input channel CH
1, CH
2, CH
N along the second electric line L- of said DC input channel.
[0051] According to the invention, the DC section 1A of the inverter 1 comprises a measuring
circuit arrangement for measuring the isolation resistance R
ISO of the photovoltaic strings 200, when said inverter operates at predefined test conditions.
[0052] For the sake of clarity, it is specified that the term "isolation resistance" identifies
the overall resistive impedance R
ISO towards ground of the photovoltaic strings 200, which is substantially given by the
parallel of the single resistive components R
1, R
2 R
N of the impedances towards ground of the single photovoltaic strings 200.
[0053] For the sake of clarity, it is specified that, when it operates under said predefined
test conditions, the inverter 1 carries out no power conversion functionalities.
[0054] Thus, when the inverter 1 operates under said predefined test conditions:
- each DC/DC converter CV1, CV2, CVN of the DC section 1A is deactivated and has its active elements (switches S1 and
S2) in an interdiction state;
- the DC/AC converters of the DC/AC conversion section 1B are deactivated and have their
active elements (switches) in an interdiction state;
- a DC current, which is fed by the corresponding photovoltaic string 200, may flow
along the first and second electric lines L+ and L- of each DC input channel CH1, CH2, CHN.
[0055] As it may be noticed, in the embodiments of figures 2 and 2A, when inverter 1 operates
under said test conditions, a DC current may flow along the electric line L+ and L-
of the corresponding DC input channel CH
1, CH
2, CH
N only.
[0056] The switches S1 and S2 of each DC/DC converter CV
1, CV
2, CV
N are in an interdiction state and are equivalent to open circuits.
[0057] The first intermediate node N1 of the first electric line L+ is substantially equipotential
(namely short-circuited, apart the presence of possible parasitic resistances) with
the first input terminal IT+ whereas the second intermediate node N2 of the second
electric line L- is substantially equipotential (namely short-circuited, apart the
presence of possible parasitic resistances) with the second input terminal IT-. The
first and second inductances L1, L2 are substantially equivalent to short-circuits
as their impedance is virtually null for very low or null operating frequencies.
[0058] No DC currents flow along the input capacitance C0 and the output capacitances C1
and C2. These latter which are substantially equivalent to open circuits as their
impedance is virtually infinite for very low or null operating frequencies.
[0059] According to the invention, the above-mentioned measuring circuit arrangement comprises
a switch circuit SW
1, SW
2, SW
N for each DC input channel CH
1, CH
2, CH
N.
[0060] Each switch circuit SW
1, SW
2, SW
N is electrically connected with a common electric node N0 that conveniently operates
a different voltage potentials with respect to the third input terminal T+ of the
DC/AC conversion section 1B, when the inverter 1 operates in the above-mentioned predefined
test conditions and at least one of said switch circuits is in an interdiction state.
[0061] In practice, each switch circuit SW
1, SW
2, SW
N is electrically connected with a common node N0 that is not electrically connected
in a direct matter with the third input terminal T+ of the DC/AC conversion section
1B so as to be equipotential with said third input terminal and at least one of said
switch circuits is in an interdiction state.
[0062] This solution is quite advantageous as it allows substantially improving the accuracy
of the isolation resistance measurements, which may otherwise be affected by relevant
errors, if the common node was always equipotential with the third input terminal
T+ of the DC/AC conversion section 1B, as it occurs in some known solutions of the
state of the art.
[0063] According to some embodiments of the invention, each switch circuit SW
1, SW
2, SW
N is electrically connected with the first input terminal IT+ of the corresponding
DC input channel CH
1, CH
2, CH
N or with an electric node (e.g. the first intermediate electric node N1) of the first
electric line L+ of the corresponding DC input channel CH
1, CH
2, CH
N. Said electric node N1 is short-circuited (apart the presence of possible parasitic
resistances) with the first input terminal IT+, when the inverter 1 operates at the
above-mentioned test conditions. According to alternative embodiments of the invention,
each switch circuit SW
1, SW
2, SW
N is electrically connected with the second input terminal IT- of the corresponding
DC input channel CH
1, CH
2, CH
N or with an electric node (e.g. the second intermediate electric node N2) of the second
electric line L- of the corresponding DC input channel CH
1, CH
2, CH
N. Said electric node N2 is short-circuited (apart the presence of possible parasitic
resistances) with the first input terminal IT-, when the inverter 1 operates at the
above-mentioned test conditions.
[0064] Preferably, each switch circuit SW
1, SW
2, SW
N comprises a first resistive element R
S and a third switch S
SW (e.g. a MOSFET) electrically connected in series between the first input terminal
IT+ and the common electric node N0 (figure 3).
[0065] Preferably, each switch circuit SW
1, SW
2, SW
N comprises a first driving circuit DV
SW to suitably drive the third switch Ssw in response to second control signals Csw
received in input. According to the invention, the above-mentioned measuring circuit
arrangement comprises a first detection circuit 10.
[0066] The first detection circuit 10 is adapted to provide first detection signals D1 indicative
of a first voltage V
ISO between the third input terminal T+ of the DC/AC conversion section 1B and the ground,
when the inverter 1 operates at the above-mentioned test conditions (figures 2, 2A,
4).
[0067] As it may be easily understood, the first voltage V
ISO is indicative of the voltage difference between the operative voltage of the third
input terminal T+ and the ground voltage, when the inverter 1 operates at the above-mentioned
test conditions.
[0068] Preferably, the first detection circuit 10 comprises a first resistive element R
10 and a fourth switch S
10 (e.g. a MOSFET) electrically connected in series between the third input terminal
T+ and the ground.
[0069] Preferably, the first detection circuit 10 comprises a second driving circuit DV
10 to suitably drive the fourth switch S
10 in response to third control signals C
10 received in input. Preferably, the first detection circuit 10 further comprises a
first detection output M
10, at which it makes available the first detection signals D1.
[0070] As it will better emerge from the following, the arrangement of the first detection
circuit 10 is essential to measure the isolation resistance R
ISO of the photovoltaic strings 200.
[0071] Preferably, the above-mentioned measuring circuit arrangement comprises a second
detection circuit 20 adapted to provide second detection signals D2 indicative a second
voltage V
TEST between the common electric node N0 and the fourth input terminal T- of the DC/AC
conversion section 1B, when the inverter 1 operates at the above-mentioned test conditions
(figures 2, 2A, 6).
[0072] As it may be easily understood, the second voltage V
TEST is indicative of the voltage difference between the operating voltage of the common
electric node N0 and the operating voltage of the fourth input terminal T-, when the
inverter 1 operates in the above-mentioned test conditions.
[0073] Preferably, the second detection circuit 20 comprises a second resistive element
R
20 electrically connected in series between the common electric node N0 and the fourth
input terminal T- and an operational circuit OP electrically connected in parallel
with the second resistive element R
20 (figure 6).
[0074] Preferably, the operational circuit OP comprises an operational amplifier and a suitable
polarization circuit CP electrically connected with the fourth input terminal T- to
suitably drive said operational amplifier.
[0075] Preferably, the second detection circuit 20 comprises a second detection output M
20, at which it makes available the second detection signals D2.
[0076] Preferably, the above-mentioned measuring circuit arrangement comprises a third detection
circuit 30 for each DC input channel CH
1, CH
2, CH
N.
[0077] Each third detection circuit 30 is adapted to provide third detection signals D3
indicative of a third voltage V
IN between the first and second input terminals IT+, IT- of the corresponding DC input
channel CH
1, CH
2, CH
N, when the inverter 1 operates in the above-mentioned test conditions.
[0078] As it may be easily understood, the third voltage V
IN is indicative of the voltage difference between the operating voltages of the first
and second input terminals IT+, IT- of the corresponding DC input channel CH
1, CH
2, CH
N, when the inverter 1 operates in the above-mentioned test conditions.
[0079] Preferably, each third detection circuit 30 is comprises a resistive shunt R
30 electrically connected in parallel with the first and second input terminals IT+,
IT- of the corresponding DC input channel CH
1, CH
2, CH
N (figure 5).
[0080] Preferably, each third detection circuit 30 comprises a third detection output M
30, at which it makes available the third detection signals D3.
[0081] As it will better emerge from the following, the arrangement of the second and third
detection circuits 20, 30 is quite important to check the operating status of each
switching circuit SW1, SW2, SWN, preferably before carrying out a measurement of the
isolation resistance R
ISO of the photovoltaic strings 200.
[0082] According to the invention, the inverter 1 comprises a control section 1D operatively
associated with the DC section 1A, the DC/AC conversion section 1B and the AC section
1C to control the operation of these latter.
[0083] The control section 1D may include one or more control boards or units, each of which
may include one or more digital processing devices (e.g. microcontrollers, DSPs, and
the like) and suitable electronic circuits of digital or analog type.
[0084] Conveniently, the control section 1D comprises suitable data processing means to
carry out its functionalities, some of which (the first, second and third processing
means 51, 52, 53) will be better described in the following.
[0085] If implemented in a digital manner, said data processing means may comprise suitably
arranged software instructions stored in a medium and executable by one or more digital
processing devices of the control section 1D to perform said functionalities.
[0086] If implemented in analog manner, said data processing means may comprise electronic
circuits suitably arranged to perform said functionalities.
[0087] Additional variant solutions to implement said data processing means are available
to the skilled person.
[0088] Preferably, the control section 1D is operatively coupled with the first and second
switches S1, S2 of each DC/DC converter CV
1, CV
2, CV
N and it is capable to provide first control signals (not shown) to control the operation
of these latter switches.
[0089] Conveniently, the control section 1D is operatively coupled with each switch circuit
SW
1, SW
2, SW
N and it is capable to provide second control signals Csw to control the operation
of the third switch S
SW of each switch circuit SW
1, SW
2, SW
N.
[0090] Conveniently, the control section 1D is operatively coupled with the above-mentioned
first detection circuit 10 (namely with the first detection output M
10 and second driving circuit DV
10 thereof) to receive the first detection signals D1 and to provide third control signals
C
10 to control the operation of the fourth switch S
10.
[0091] Preferably, the control section 1D is operatively coupled with the above-mentioned
second detection circuit 20 (namely with the second detection output M
20 thereof) to receive the second detection signals D1.
[0092] Preferably, the control section 1D is operatively coupled with the above-mentioned
third detection circuit 30 (namely with the third detection output M
30 thereof) to receive the third detection signals D3.
[0093] Preferably, the control section 1D comprises first data processing means 51 adapted
to carry out MPPT (Maximum Power Point Tracking) functionalities by suitably controlling
the DC/DC converters CV
1, CV
2, CV
N of the input channels CH
1, CH
2, CH
N and the DC/AC converters of the DC/AC conversion section 1B.
[0094] In order to carry out said MPPT functionalities, the first data processing means
51 may implement algorithms or procedures of known type that will be not described
in the following for the sake of brevity.
[0095] Preferably, the control section 1D comprises second data processing means 52 configured
to carry out a test procedure P
T adapted to test the operating conditions of a switch circuit SW
1, SW
2, SW
N operatively associated with a given DC input channel CH
1, CH
2, CH
N of the DC section 1A.
[0096] The test procedure P
T will now be described in details in accordance to a preferred embodiment of the invention.
[0097] The test procedure P
T comprises a step of commanding the inverter 1 to take or maintain the above-mentioned
predefined test conditions.
[0098] In order to implement this step, the second data processing means conveniently provides
suitable control signals to the switches S1, S2 of each DC/DC converter CV
1, CV
2, CV
N and suitable control signals to the DC/AC converters of the DC/AC conversion section
1B to maintain said switches i an interdiction state.
[0099] The test procedure P
T then comprises the step of selecting an input channel (for example the input channel
CH
1 - figures 2, 2A).
[0100] The test procedure P
T then comprises the step of commanding the switching circuit SW
1 operatively associated with the selected input channel CH
1 to switch in a conduction state and the step of commanding the switching circuits
SW
2, SW
N operatively associated with the input channels CH
2, CH
N different from the selected input channel CH
1 to switch in an interdiction state.
[0101] In order to implement this step, the second data processing means conveniently provides
suitable control signals Csw to the driving circuits DV
SW of the switching circuits SW
1, SW
2, SW
N in such a way that the corresponding switches S
SW are operated in an interdiction state or in a conduction state.
[0102] Following the above-mentioned steps, the common node N0 becomes substantially equipotential
with the first input terminal IT+ of the selected DC input channel CH
1 and its decoupled from the first input terminals IT+ of the remaining DC input channels
CH
2, CH
N. The test procedure P
T then comprises the step of receiving the second detection signals D2 from said the
detection circuit 20 and the step of receiving the third detection signals D3 from
said third detection circuit 30.
[0103] The second detection signals D2 include one or more first voltage values V
A indicative of the detected second voltage V
TEST whereas the third detection signals D3 include one or more second voltage values
V
B indicative of the detected third voltage V
IN.
[0104] The test procedure P
T then comprises the step of comparing the above-mentioned first and second voltage
values V
A, V
B.
[0105] As the common node N0 is substantially equipotential with the first input terminal
IT+ of the selected DC input channel CH
1, the second and third voltages V
TEST and V
IN should have similar first and second voltage values V
A, V
B, if the switching circuit SW
1 operatively associated with the selected input channel CH
1 works properly. Otherwise, a substantial voltage difference between the first and
second voltages V
A, V
B will be certainly present.
[0106] The test procedure P
T then comprises the step of determining a fault condition of the switching circuit
SW
1 operatively associated with the selected input channel CH
1, if a voltage difference value between the second and third second voltages V
TEST, V
IN exceeds a given voltage threshold value V
TH.
[0107] In practice, a fault condition of the switching circuit SW
1 operatively associated with the selected input channel CH
1 is determined if the following condition occurs:
where V
A is a first voltage value of the detected second voltage V
TEST, V
B is a second voltage value of the detected third voltage V
IN and V
TH is the above-mentioned threshold value. Preferably, the second data processing means
52 are configured to carry out the above-described test procedure P
TEST for each DC input channel CH
1, CH
2, CH
N.
[0108] At each execution cycle of the test procedure P
T, the second processing means 52 will conveniently select a different DC input channel
CH
1, CH
2, CH
N to check the actual operative status of the corresponding switching circuit SW
1, SW
2, SW
N.
[0109] It is evidenced that at each execution cycle of the test procedure P
T, the common electric node N0 always operates a different voltage potentials with
respect to the third input terminal T+ of the DC/AC conversion section 1B, as N-1
switch circuits among the switching circuits switching circuit SW
1, SW
2, SW
N are in an interdiction state.
[0110] The above-mentioned test procedure P
T allows testing the actual operating status of the switching circuits SW
1, SW
2, SW
N operatively associated with the DC input channels CH
1, CH
2, CH
N.
[0111] Conveniently, the actual operating status of the switching circuits SW
1, SW
2, SW
N may be checked by carrying above-mentioned test procedure P
T before measuring the isolation resistance R
ISO of the photovoltaic strings 200
[0112] In this way, relevant measurement errors of the isolation resistance R
ISO of the photovoltaic strings 200 may be preventively avoided with relevant advantages
in terms of reliability and accuracy.
[0113] Preferably, the control section 1D comprises third data processing means 53 configured
to carry out a measuring procedure P
M adapted to measure the isolation resistance R
ISO of the photovoltaic strings 200.
[0114] The measuring procedure P
M will now be described in details in accordance to a preferred embodiment of the invention.
[0115] The measuring procedure P
M comprises a step of commanding the inverter 1 to take or maintain the above-mentioned
predefined test conditions.
[0116] In order to implement this step, the second data processing means conveniently provides
suitable control signals to the switches S1, S2 of each DC/DC converter CV
1, CV
2, CV
N and suitable control signals to the DC/AC converters of the DC/AC conversion section
1B to maintain said switches in an interdiction state.
[0117] The measuring procedure P
M comprises a step of commanding the first switch S
10 of the first detection circuit 10 to switch in an interdiction state.
[0118] In order to implement this step, the third data processing means 53 conveniently
provides suitable control signals C
10 to the driving circuits DV
10 of the switch S
10.
[0119] The measuring procedure P
M comprises a step of receiving the first detection signals D1 indicative of the first
voltage V
ISO. The first detection signals D1 include one or more third voltage values V
C indicative of the detected first voltage V
ISO.
[0120] The measuring procedure P
M then comprises a step of commanding the first switch S
10 of the first detection circuit 10 to switch in a conduction state.
[0121] In order to implement this step, the third data processing means 53 conveniently
provides suitable control signals C
10 to the driving circuits DV
10 of the switch S
10.
[0122] The measuring procedure P
M comprises a further step of receiving the first detection signals D1 indicative of
the first voltage V
ISO. The first detection signals D1 include one or more fourth voltage values V
D indicative of the detected first voltage V
ISO.
[0123] The measuring procedure P
M then comprises a step of calculating the isolation resistance R
ISO of the photovoltaic strings 200 basing on the third and fourth voltage values V
C, V
D.
[0124] In order to calculate the isolation resistance R
ISO, the third processing means 53 may adopt known algorithms of know type.
[0125] As an example, the isolation resistance R
ISO may be calculated as:
where V
C is a third voltage value of the detected first voltage V
ISO with the switch S
10 in interdiction state, V
D is a third voltage value of the detected first voltage V
ISO with the switch S
10 in conduction state and R
10 is the known resistance value of the first resistive element R
10 of the first detection circuit 10.
[0126] The measuring procedure P
M allows measuring the isolation resistance of the photovoltaic strings 200 in a simple
manner by carrying out subsequent measurements of a single circuit point (the third
input terminal T+ of the DC/AC conversion section 1B) with respect to the ground.
[0127] This constitutes an important advantages with respect to known MPPT inverters the
state of the art where the voltages of multiple points of the DC section have to be
measured to calculate the isolation resistance of the photovoltaic strings.
[0128] The present invention allows achieving the intended aims and objects.
[0129] The inverter 1 is provided with a measuring circuit arrangement that allows measuring
the isolation resistance R
ISO of the photovoltaic strings 200 in an accurate and reliable manner. The measuring
circuit arrangement employed in the inverter 1 is particularly adapted for use in
an inverters is capable of providing MPPT functionalities and having the DC input
channels CH1, CN2, CHN without equipotential lines in common.
[0130] The inverter 1 has a compact structure with a relatively small size. The above-mentioned
measuring circuit arrangement may in fact be easily integrated in the DC section 1A
and it may employ MOSFETs or equivalent switching devices for its practical implementation.
[0131] The inverter 1 can be easily manufactured at industrial level with highly automated
operations at competitive costs with respect to known solutions of the state of the
art.
1. An inverter (1) for a photovoltaic apparatus (100), said inverter comprising a DC
section (1A), a DC/AC conversion section (1B), an AC section (1C) and a control section
(1D), wherein said DC section includes a plurality of DC input channels (CH
1, CH
2, CH
N), each DC input channel comprising:
- first and second input terminals (IT+, IT-) adapted to be electrically connected
with a corresponding photovoltaic string (200);
- first and second output terminals (OT+, OT-) electrically connected with a third
input terminal (T+) and a fourth input terminal (T-) of said DC/AC conversion section
(1B), respectively;
- a first electric line (L+) including said first input and output terminals (IT+,
OT+) and a second electric line (L-) including said second input and output terminals
(IT-, OT-);
- a DC/DC converter (CV1, CV2, CVN) electrically connected between said input terminals and said output terminals;
characterised in that said DC section (1A) comprises a measuring circuit arrangement adapted to measure
an isolation resistance (R
ISO) of said photovoltaic strings (200), when said inverter operates in predefined test
conditions, said measuring arrangement comprising:
- for each DC input channel (CH1, CH2, CHN), a switch circuit (SW1, SW2, SWN) electrically connected with a common electric node (N0) operating at a different
voltage potentials with respect to the third input terminal (T+) of said DC/AC conversion
section (1B), when said inverter operates in said test conditions and at least one
of said switch circuits is in an interdiction state, said switch circuit (SW1, SW2, SWN) being further electrically connected with:
- said first input terminal (IT+) or an electric node (N1) of said first electric
line (L+) that is short-circuited with said first input terminal (IT+), when said
inverter operates in said test conditions; or
- said second input terminal (IT-) or an electric node (N2) of said second electric
line (L-) that is short-circuited with said second input terminal (IT-), when said
inverter operates in said test conditions;
- a first detection circuit (10) adapted to provide first detection signals (D10) indicative of a first voltage (VISO) between the third input terminal (T+) of said DC/AC conversion section (1B) and
the ground, when said inverter operates in said test conditions.
2. Inverter, according to claim 1, characterised in that said first detection circuit (10) comprises a first resistive element (R10) and a switch (S10) electrically connected in series between the third input terminal (T+) of said DC/AC
conversion section (1B) and the ground.
3. Inverter, according to one or more of said previous claims, characterised in that said measuring circuit arrangement comprises a second detection circuit (20) adapted
to provide second detection signals (D2) indicative a second voltage (VTEST) between said common electric node (N0) and the fourth input terminal (T-) of said
DC/AC conversion section (1B), when said inverter operates in said test conditions.
4. Inverter, according to claim 3, characterised in that said second detection circuit (20) comprises a second resistive element (R20) electrically connected in series between said common electric node (N0) and the
fourth input terminal (T-) of said DC/AC conversion section (1B) and an operational
circuit (OP) electrically connected in parallel with said second resistive element
(R20).
5. Inverter, according to one or more of the previous claims, characterised in that said measuring circuit arrangement comprises, for each DC input channel (CH1, CH2, CHN), a third detection circuit (30) adapted to provide third detection signals (D3)
indicative of a third voltage (VIN) between the first and second input terminals (IT+, IT-) of said DC input channel,
when said inverter operates in said test conditions.
6. Inverter, according to claim 5, characterised in that said third detection circuit (30) comprises a resistive shunt (R30) electrically connected in parallel with the first and second input terminals (IT+,
IT-) of said DC input channel (CH1, CH2, CHN).
7. Inverter, according to one or more of the previous claims, characterised in that said control section (ID) comprises first data processing means (51) configured to
carry out MPPT functionalities.
8. Inverter, according to one or more of the previous claims,
characterised in that said control section (ID) comprises second data processing means (52) configured
to carry out a test procedure (P
T) adapted to test operating conditions of a switch circuit (SW
1, SW
2, SW
N) operatively associated with a DC input channel (CH
1, CH
2, CH
N), said test procedure including the following steps:
- commanding said inverter (1) to take or maintain said predefined test conditions;
- selecting an input channel (CH1);
- commanding the switching circuit (SW1) operatively associated with the selected input channel (CH1) to switch in a conduction state;
- commanding the switching circuits (SW1, SW2, SWN) operatively associated with the input channels (CH2, CHN) different from the selected input channel (CH1) to switch in an interdiction state;
- receiving said second detection signals (D2) including one or more first voltage
values (VA) indicative of said second voltage (VTEST);
- receiving said third detection signals (D3) including one or more second voltage
values (VB) indicative of said third voltage (VIN);
- comparing said second voltage values (VA) with said third voltage values (VB);
- determining a fault condition of the switching circuit (SW1) operatively associated with the selected input channel (CH1), if a voltage difference value between said second and third second voltage values
(VA, VB) exceeds a given voltage threshold.
9. Inverter, according to claim 8, characterised in that said second data processing means are configured to carry out said test procedure
(PTEST) for each DC input channel (CH1, CH2, CHN).
10. Inverter, according to one or more of the claims from 2 to 9,
characterised in that said control section(1D) comprises third data processing means configured to carry
out a measuring procedure (P
M) to measure the isolation resistance (R
ISO) of said photovoltaic strings (200), said detection procedure including the following
steps:
- commanding said inverter (1) to take or maintain said predefined test conditions;
- commanding the first switch (S10) of said first detection circuit (10) to switch in an interdiction state;
- receiving said first detection signals (D1) including one or more third voltage
values (Vc) indicative of said first voltage (VISO);
- commanding the first switch (S10) of said first detection circuit (10) to switch in a conduction state;
- receiving said first detection signals (D1) including one or more fourth voltage
values (VD) indicative of said first voltage (VISO);
- calculating said an isolation resistance value (RISO) basing on said third and fourth voltage values (VC, VD).
11. Inverter, according to one or more of the previous claims, characterised in that said DC/DC converter (CV1, CV2, CVN) has a two-level boost configuration.
12. A photovoltaic apparatus (100) characterised in that it comprises an inverter (1), according to one or more of the previous claims.